1. Field of the Invention
The present invention generally relates to monitors for detecting cardiac and pulmonary motions such as heartbeat, respiration, physical movement, and other body activities of a patient.
2. Description of Related Art
For detecting vital signs of a patient, there are a number of monitors available with a variety of uses and applications. These vital sign monitors and other advanced monitors range from simple sound monitors such as xe2x80x9cbaby monitorsxe2x80x9d used at home, to sophisticated apnea monitors and electro-cardiograms (EKGs) used by physicians and hospitals.
Off-the-shelf low cost, non-prescription baby monitors are currently in wide use to monitor infants at home to verify an infant""s well-being. These monitors generally employ a microphone sensor that sends a signal to a remote unit with visual and audio displays. For example, the microphone sensor can be placed close to the crib on a table or hooked up to the crib with a bracket while the remote unit is carried around in the house by parent or other caregiver. Although these units are suitable to monitor low risk infants when they are awake and moving around or crying in the crib, they are not reliable for monitoring them during sleep periods. Parents may be concerned that their child is in jeopardy if they can not hear their child""s cries or other normal sounds over the monitor. To address this concern, a parent may make many visits to the room where the infant is asleep to either wait patiently until the infant moves in the crib or attempt to touch the infant in an effort to detect child""s breathing.
An alternative system for monitoring infants on the market today uses a rocker switch or pressure/capacitive sensor placed under a mattress to detect the rocking action of the mattress when the infant or other patient moves. To work, such a system requires a hard surface under the mattress and a rocking mechanism placed between the mattress and the hard surface. When the patient moves, the mattress rocks back and forth across a fulcrum causing the sensor to detect the movement. The presence of the right amount of movement might indicate that the patient is healthy, while the lack of movement or too much movement could suggest that the patient is in trouble; although lack of movement does not always reflect that the patient is in trouble. In the case of a small infant who normally does not move much, the sensor may not detect any movement, and accordingly it may produce a large number of false alarms. A significant number of false alarms will undoubtedly cause the parent or caregiver to stop using the device and render it effectively useless. Additionally, the system is not sensitive enough to accurately detect the small-scale acousto-mechanical signals generated by the heart beating, which may provide a complimentary signal that could be used to reduce false alarms.
Neonatologists use apnea monitors for detecting sleep apneic events in premature or high-risk infants, which if not detected can result in Sudden Infant Death Syndrome (SIDS). Generally the apnea monitors are used to detect and measure respiration rate of an infant. In the absence of respiration for about 20 seconds, the monitor sounds an alarm indicating an urgent need for nurse intervention. These type of monitors typically utilize impedance pneumography techniques, in which the electrical sensors are placed on infant""s body to detect variations in resistance caused by respiration and physiologic changes when a 10-100 kHz electrical current of about 50 micro amperes is passed through the patient. The voltage developed and measured is proportional to the transthoracic impedance, which varies during each breath. These sensors are designed to measure changes in this resistance, on the order of 0.1-1 ohms for shallow breather infants to normal ones. One problem with these sensors is that they do not provide high enough sensitivity to distinguish between shallow breathing and no breathing. As a result, they generate a large number of false alarms, requiring constant attention of nurses and frequent re-setting of the instrument. To better detect the apneic event, a separate heart rate sensor is often added to apnea monitors. Monitoring the heart rate in conjunction with respiration can lower the false alarm rates because prolonged apnea is frequently followed by bradycardia (i.e. slowing of the heart rate), which is an event that can be measured using the heart rate sensor. Further improvements can be made by incorporating a saturated oxygen monitoring sensor to an apnea monitor. Such a sensor is affixed at the finger tip of the patient using separate electrical leads. Monitoring the changes in oxygen level in conjunction with respiration and heartbeat rates is useful because it can more accurately detect the onset of an apenic event so that appropriate decisions can be made to administer the correct treatment.
Even though addition of both a heart rate sensor and an oxygen sensor to an apnea monitor is useful to reduce false alarms, and better detect apneic events, such monitors still have a number of shortcomings. For example, 1) with the addition of these sensors and associated electronics, the system""s cost increases; 2) electrical leads can come loose or break; 3) if the sensors are inadvertently pulled due to body motion or lead tangling, the infant""s skin can peel off; 4) the infant""s skin can develop dermatitis from electrode creams, gels and adhesives 5) because the sensors are electrical in nature, they are susceptible to interference from electromagnetic sources, static discharge, radio frequencies, and shocks; 6) they do not function properly in the presence of conductive liquids; 7) they utilize a number of sensors and leads to pick up signals from the patients, which can impede the development of mother/infant relationship; 8) unless the respiration sensors are appropriately placed and the controls adjusted properly, the monitor can be sensitive to artifacts generated by heartbeat; 9) patients can not sleep well because they are forced to lie in a certain position determined by the sensors, and cannot move around freely once the sensors and leads are hooked up; 10) if X-rays are required, the sensors must be removed; and 11) the output signals are susceptible to body motion artifacts.
Another type of monitor is a motion sensing/non-impedance type, such as an air mattress that is designed and developed for the detection of sleep apnea. The motion-sensing sensors are based on the principle of air displacement from one segment to another through a manifold, which is measured by a heated thermistor in the manifold. The moving within the mattress air cools the heated thermistor in the manifold, and this temperature change is detected as a breath. Unfortunately, these pads are very sensitive to body motion and vibration artifact, and therefore the respiration signals are not easily detected from the artifacts. In addition, these sensor pads are not rugged enough to fold and flex, and therefore, are difficult to use and store. Furthermore, because the sensor includes an electrically active thermistor, it is prone to electromagnetic interference.
Other monitor types, including sensors with capacitive mattresses, and sensors with magnetic, thermistor, and pressure transducer pads have similar disadvantages. Because most devices used to date are electrical in nature, they have similar shortcomings as described for the impedance or air mattress sensors. None are totally suitable for reliably detecting apneic conditions or near-miss SIDS.
U.S. Pat. No. 5,241,300 discloses a fiber optic breathing sensor that monitors the air movement localized at the nasal cavities. By placing this fiber optic sensor at the base of the nasal cavity, it measures the change in temperature due to air flow. This sensor does not provide the cardiac signal necessary in the clinical diagnosis of the apnea. Although this system may be useful as a supplementary sensor in the detection of central apnea in adults, it cannot be used with infants because it requires a relatively large volume of air displacement for proper operation.
U.S. Pat. Nos. 5,291,013 and 5,212,379 disclose a Fiber Optical Monitor For Detecting Motion Based on Changes in Speckle Patterns. This speckle-based monitor is designed to detect breathing and heart beat when generally a coherent, narrow band light source is launched at one end of a multimode optical fiber. The central part of the fiber is formed into a xe2x80x9cblanketxe2x80x9d that senses a patient""s normal breathing and heart beat motion. The system, using a photo-detector at the other end of the fiber that senses the changes in optical intensity of a 2-dimensional (2D) speckle pattern. The photo-detector""s amplitude output signal is related to the integration (averaging) of the number of speckles and its time variation is related to the change or re-distribution in the number of speckles. Using a coherent optical source and a multimode fiber, the speckle-based monitor produces a light cone containing a pattern of varying light intensity at the fiber output generating multiple light and dark grainy spots (a speckle pattern). The speckle-based monitor requires that only a certain fraction of the light cone emitting from the fiber output (i.e. less than all of the emitted light cone) is collected by the photo-detector or two halves of a photo detector. This is necessary because if entire cross-section of the light cone, containing the 2D speckle pattern, is collected by the photo-detector (or two detectors halves), its output will not appreciably change when motion signals such as breathing or heartbeat, are coupled to the fiber.
The 2D speckle pattern is produced primarily due to the multiple intermodal interferences between the propagating spatial modes of the fiber, and interferences caused by the coherent beams mixing on the receiving surface. The pattern produced by the intermodal interference is modulated by the motion signals that are coupled to the fiber, whereas the pattern produced by the receiving surface remains unaffected. The speckle-based fiber monitor thus relies only on the intermodal interferences to detect the motion signals such as breathing and heartbeat.
In the speckle-based monitor, generally a multi-mode fiber is required over a single mode fiber to detect the patient""s motion. Without the multi-mode fiber, it is difficult to produce measurable speckle pattern on the optically polished surface of the detector (or two halves) using even a highly coherent light source. This is because specular surfaces can not produce sufficient optical phase shifts required for the coherent light beams to interfere. Single mode fibers do not show intermodal speckle patterns because the interference of the two orthogonal degenerate mode only produces a single light spot, compared to may spots by multi-mode fibers. Higher modes in a single mode fiber may be generated if the operating wavelength of the light source is brought near or below the cut off wavelength of the fiber. These higher order modes may interfere but the speckle visibility is poor due to the presence of only few modes, resulting in poor signal detection and low sensitivity. In addition, the speckle-based monitor requires a coherent light source because incoherent light sources, such as light emitting diodes (LEDs), can not produce a measurable intermodal speckle pattern even if a multimode fiber is used. Therefore, both a coherent light source and a multi-mode optical fiber are needed in the operation of the speckle-based monitor. If compromises are made, the monitor may not be provide sufficient sensitivity to detect and measure patient""s motion signals, especially the heart signals.
The speckle-based monitor has a number of shortcomings and limitations. Such limitations can become large disadvantages when design trade-offs are made. For example, the requirement that the light source be coherent and narrow band, limits the laser source choices to gas or distributed feedback (DFB) lasers, which results in high power or high costs requirements. Unfortunately, gas lasers such as Helium-Neon (Hexe2x80x94Ne) require high voltages to create the plasma and therefore are not very desirable for use at home or in hospitals where patient safety is paramount. DFB lasers, on the other hand are expensive and therefore less desirable in applications where low cost is important. Less coherent (and less costly) broadband light sources, such as edge light emitting diodes (ELEDs) or laser diodes (LDs) that can be operated at low voltages, are not suitable for speckle-based sensors because they do not generate detectable speckle pattern, with or without a multi-mode optical fiber. Another limitation of the speckle-based monitor is the requirement to place the end of the fiber from the detector at sufficient separation to collect a certain fraction of the speckle cross-sectional area. This requirement affects the detection sensitivity in addition to introducing manufacturing difficulties and correspondingly poor yields. Other limitations of the speckle sensor are: 1) since the fiber is placed as a blanket over the patient, it may not stay over the patient at all times especially when the patient is not attended; 2) in order for the speckle-based monitor to provide adequate sensitivity, it must use multi-mode fibers rather than single mode; and 3) use of simple bandpass or high pass filter designs in the speckle-based monitor are insufficient to reliably extract the heart beat signal from the respiration signal in the presence of body movement. The body""s movement can produce noise with frequency components in either signal bands. Since hand, leg or body movements occur typically during apneic events or otherwise, therefore common mode rejection techniques, band pass filters or split detectors can not easily eliminate these unwanted signals.
To overcome the limitations, shortcomings and disadvantages of prior art, the present invention provides a novel fiber optic monitor that utilizes optical phase interferometry to monitor a patient""s vital signs. A number of advantages are provided by using optical phase interferometry for detecting heartbeat, respiration and physical body movement. For example, because these sensors utilize optical phase as compared optical intensity modulation characteristics, they provide a very high detection sensitivity while advantageously needing only single mode fibers. Single mode fibers are widely available, and are much lower in cost compared to multimode fibers. The monitor can be configured with a single sensor and in some embodiments an array of sensors to simultaneously detect physiological parameters such as pulmonary motion, cardiac activity, physical movement, and other body activities in infants, adults, canines, and any other living creature.
Because the system is non-invasive, passive and free from electrical leads, gels and suction cups, it can reliably monitor heart beat and respiration rates in presence of physical movement while remaining virtually transparent to the patient. It has application for the detection of sleep apnea and cardiac disorders in both infants and adults. In addition, it can quantitatively predict the body""s physical parameters such as fat/weight ratio, temperature, etc. In some embodiments of the monitor, the system can be made portable so that the patient can walk around while still being continuously monitored. Advantageously, some embodiments of the monitor utilize low cost, broadband sources of optical radiation which are typically safer and more compact providing lower cost, smaller size and weight.
A fiber optic, interferometric monitor for detecting vital functions in a patient comprises an optical fiber interferometer that generates an optical signal responsive to acousto-mechanical signals generated by vital functions of the patient. The interferometer includes optical fiber means for defining two optical paths including a first optical path and a second optical path. The first optical path includes an optical fiber proximately situated to the patient so that the acousto-mechanical signals are coupled to the optical fiber, thereby modulating a physical parameter of the optical fiber responsive to the acousto-mechanical signals. The optical fiber also includes an optical source coupled to supply optical radiation into an input end of the first and second optical paths. The interferometer generates a serial train of fringes from the optical radiation emitted from an output end of the two optical paths. A photo-detector is arranged to sense an optical signal provided by time variations in the fringe train, the photo-detector providing a raw electrical signal responsive thereto. A signal processor is coupled to the optical detector to process the raw electrical signal to provide one or more processed output signals indicative of the vital functions. An output system is provided for communicating the one or more processed signals.
The optical fiber sensor (or a plurality of sensors) is situated in close proximity to a patient, for example a length of optical fiber may be configured into a pad, which can be placed on top of a mattress but under the bed sheet. In such a configuration, the optical fiber sensor (or sensors) is non-invasive, it does not need to be hooked up to a patient using gels or suction cups or leads, and therefore it remains virtually unnoticed by the patient, allowing the patient to rest comfortably.
The monitor system has applicability across a broad spectrum of fields, from routine monitoring of infants at home to apnea, and arrhythmia that occur in sleep labs, emergency rooms, operating rooms intensive care units (ICU) and rescue and ambulatory vehicles. The system can be implemented in a wide variety of embodiments ranging from a low cost in-home baby monitor to a high end apnea monitor for in hospital use to monitor infants in nenonatology laboratories or at home by physician""s prescription. The monitor can be used for providing additional and complimentary ballisto-mechanical information to be combined with EKG information for early diagnosis or prediction of cardiac conditions or events. If it is used to monitor respiratory rate, an increase may indicate acidotic condition as might be seen with septic shock or diabetic ketoacidosis. The monitor may also be used for monitoring adults in convalescent homes for the detection of vital signs or in psychiatric wards to detect presence of patients in bed. For military applications, the monitor may be used to monitor physiological and pathological changes in personnel under various combat or training conditions as well as in trauma facilities.